The demand for hydrogen is predicted to increase sharply in various industrial fields. With such a background, development of a hydrogen-permeable membrane usable for purification of hydrogen is in progress. As the hydrogen-permeable membrane, there is known one using palladium (Pd). Pd is a rare noble metal and is very expensive.
Hence, development of a substitute material which is less expensive than Pd. For example, there was proposed, in the claim of JP-A-2002-336664, a hydrogen-permeable membrane unit obtained by forming a hydrogen-permeable membrane composed of niobium (Nb), vanadium (V), tantalum (Ta), etc., on a porous substrate having gas permeability. There was proposed, in the claim of JP-A-1995-000775, a hydrogen-permeable membrane using an alloy of zirconium (Zr) with nickel, chromium, iron, copper, vanadium, titanium, etc.
However, substitute materials for Pd are, in general, inferior to Pd in hydrogen permeability. Further, the substitute materials react with hydrogen and resultantly powder, and show such deterioration. Therefore, they are inferior to Pd in durability.
Moreover, hydrogen-permeable membranes using Pd or a substitute material as a hydrogen-permeable material have a membrane thickness of about several microns to several millimeters and only hydrogen-permeable membranes having such a very large thickness have been developed heretofore. In particular, a hydrogen-permeable membrane using only Pd as a membrane material has a low mechanical strength and accordingly has a problem that the thickness can not be reduced further. Since hydrogen-permeable materials and, in particular, Pd is very expensive, a hydrogen-permeable membrane of large thickness composed of such a material becomes a high cost.
Thus, there is currently no hydrogen-permeable membrane which can satisfy three elements of performance, durability and cost and it is strongly desired to develop such a hydrogen-permeable membrane.